Which of the following is a key requirement of a protective relay in terms of reliability?

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Question

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RRB JE EE 22 Apr 2025 Shift 2 CBT 2 Official Paper

The relay must operate only when a fault occurs and remain stable during normal conditions
The relay must have a complex design to handle multiple fault types.
The relay must reset immediately after a fault is cleared.
The relay must operate with a significant time delay to ensure coordination.

Answer 1

Protective Relay

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.
The fundamental reliability requirement of a protective relay must be its operation when a fault occurs and remain stable during normal conditions.

Fundamental requirements of the protective relays:

Selectivity

The protective system can select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system.
A relay should also be able to suggest whether or not the fault is in its jurisdiction.

Speed

The relay system should disconnect the faulty section as fast as possible for the following reasons:

Critical clearing time should not be exceeded.
The electrical apparatus may be damaged if they are made to carry fault currents for long.
A persistent fault will lower the voltage, resulting in crawling and overloading of industrial drives.

Sensitivity

The relay system can operate with a low value of actuating quantity.
The sensitivity of a relay is a function of the volt-amperes(VA) input to the coil of the relay necessary to cause its operation.

Reliability

The Protective Relay system can operate under pre-determined conditions.
Without reliability, the protection would be rendered largely ineffective and could even become a liability.